1. Field of the Invention
The present invention relates to a lead frame and a semiconductor device in which a semiconductor chip is mounted on the lead frame and, more particularly, to techniques suitably applicable to a semiconductor device lead frame used to mount a resin-encapsulated semiconductor device and a semiconductor device using the lead frame.
2. Description of the Related Art
Generally, a semiconductor device in which a semiconductor chip is die-bonded to an island of a lead frame uses a lead frame processed so that the upper surface of the island is lower by a predetermined thickness than the upper surfaces of the inner leads. The lead frame is thus processed principally to prevent a bonding wire connecting the upper surface of an electrode formed on a semiconductor chip and the upper surface of an inner lead of the lead frame from contacting the edge of the semiconductor chip. It is preferable to decrease any level difference in the direction of thickness of the semiconductor chip between the electrode upper surface and the inner lead upper surface.
For example, FIG. 7A is a plan view showing a lead frame used in a so-called SOP (Small Outline L-Leaded Package) semiconductor device in which outer leads extend in two directions from the package main body. FIG. 7B is a sectional view taken along a line VI-VI' of FIG. 7A. Note that a plurality of such lead frames are continuously formed in a single frame so that a plurality of semiconductor chips can be mounted. FIG. 7A shows one of these lead frames corresponding to one semiconductor chip.
As shown in FIG. 7A, a nearly rectangular device hole 21 is formed in a central portion of the lead frame. In this device hole 21, a flat, essentially rectangular island 22, inner leads 23, outer leads 24, tiebars 25, and a pair of suspension pins 27 are formed. The inner leads 23 are aligned at predetermined pitches to extend toward a pair of opposing edges of the island 22. The outer leads 24 are connected in a one-to-one correspondence with the inner leads 23. Each tiebar 25 is formed to cross the boundary between the inner leads 23 and the outer leads 24. The suspension pins 27 connect those two edges of the island 22, which do not oppose the end portions of the inner leads 23, and a base 26. The lead frame usually has a thickness T1 of about 0.2 mm.
A bent portion 29 is formed in a predetermined portion of each suspension pin 27. Consequently, as shown in FIG. 7B, the island 22 is offset a distance Td from the base 26. This offset Td is usually set to an amount corresponding to the thickness of the semiconductor chip used. The offset Td is generally around 0.3 to 0.5 mm although it also depends upon the type of product.
FIG. 8 is a flow chart showing a process of mounting a semiconductor chip on the island 22 of the lead frame. First, several tens of lead frames to about a hundred lead frames are stacked to improve the working efficiency (step S21). The stacked lead frames are supplied to an automatic semiconductor chip mounter (step S22). In the mounter, a suction pad is lowered toward the lead frames, and the upper surface of the base 26 of the topmost lead frame is chucked by the suction pad (step S23). The suction pad is raised to separate the topmost lead frame (step S24). The upper surface of the island 22 of the separated lead frame is coated with an adhesive such as silver paste (step S25). A semiconductor chip is placed on the surface of the island 22 coated with the silver paste, and the island 22 and the chip are adhered (step S26). Through these steps the semiconductor chip is mounted on the island 22 of the lead frame.
Between steps S21 and S24 of the above process, vibrations sometimes act on the stacked lead frames which then become displaced from each other. If this is the case, in a direction parallel to the direction in which the suspension pins extend, the bent portion of the suspension pin functions as a stopper to prevent displacement in this direction. However, nothing functions as a stopper in a direction perpendicular to the extending direction of the suspension pins, i.e., in a direction parallel to the direction in which the inner leads extend. Consequently, as shown in a sectional view of FIG. 9, the island 22 of the upper one of two stacked lead frames sometimes enters between an inner lead 33 and an island 32 of the lower lead frame. If the upper lead frame is pulled up by the suction pad in this state in step S24, the inner leads 33 of the lower lead frame may deform, or the lead frame may drop from the suction pad during the pulling makeing it impossible to separate the lead frames from each other.